Chairman Mao has always been an acceptable tyrant. Compared to Hitler and Stalin, Mao Zedong has usually gotten good if not great press, despite having been responsible for the peacetime deaths of millions of his countrymen because of bad policies or purges. On the left, the view of the chairman has remained petrified in the 1930s, when the American journalist Edgar Snow wrote Red Star Over China , a flattering account of Mao and his communist comrades' fight against Japanese aggressors and Chiang Kai-shek's corrupt nationalist regime, and in the 1960s, when Mao fever swept American college campuses. (I remember going to sleep as a youngster to the chants of Columbia University students: "Mao, Mao, Chairman Mao.") From the right, Mao looked good for standing up to the Soviets in the 1970s. Generally speaking, China lovers have given Mao a break and followed the official Communist Party line: that the chairman was 30 percent wrong and 70 percent right as he united China and led tens of millions of people to their deaths.

But unlike in the West, where the view of Mao has remained petrified, scholars in China are battling over the meaning of the man. For a growing group of them, who have published their books and essays in out-of-the-way or overseas publishing houses, Mao was a tyrant. In recent years, historians writing in mainland China have made enormous contributions to the study of Mao's life -- showing how his particular brand of cruelty helped fashion China's peculiarly successful brand of totalitarianism, dissecting his debauchery and sexual deviancy, detailing his sadism, his disastrous policies, the famines he caused and the lives he destroyed. Another group has shown how Mao's crimes have devastated China's culture, creating a broken snitch society absent a moral compass.

Against that backdrop arrives the extraordinary Mao: The Unknown Story , by Jung Chang and her husband, the scholar Jon Halliday, who last collaborated on Chang's bestselling family portrait, Wild Swans . Their 814-page tome relies heavily on new Chinese scholarship; indeed, it probably could not have been written without it. The biography also benefits from solid research by Halliday in newly opened Soviet archives.

When it sticks to the new Chinese and Russian sources, the book shines, providing readers with the most detailed portrayal of the "Great Helmsman" to date. But when it pretends to tell us what the chairman is thinking and feeling, the book veers toward magical realism. Finally, its tendency toward hyperbole damages its otherwise persuasive case against Mao.

In short, if you're hoping for staid, balanced scholarship, don't read this book. It's not history; it's a screed, albeit a screed on the side of the angels. Chang writes with the zeal of the converted. As a youth, she, like millions in her generation, was intoxicated with Mao and viewed him as a god. But he ravaged untold lives, including hers. Chang obviously figured she didn't need to get mad; she got even.

Even screeds have their place, however, and this is an extremely entertaining one. Indeed, sometimes an emotionally charged account -- one written with obvious biases -- can reveal the truth better than ostentatious, morally numbed objectivity that cloaks a lot of Western scholarship on China. Chang and Halliday's point is very simple: Like a small group of scholars in China, they believe that Mao wasn't a revolutionary but a monster. He wasn't a communist but a bandit king. The result is a page-turner with a point.

And that point is honed razor-sharp. "Turning ordinary organisations into virtual prisons was a significant innovation of Mao's," they write. "Here he went far beyond anything either Hitler or Stalin achieved: he converted people's colleagues into their jailers. . . . He greatly enlarged the number of people directly involved in repression, including torture, making the orbit significantly wider than either Stalin or Hitler, who mostly used secret elites (KGB, Gestapo) that held their victims in separate and unseen locales." Chang and Halliday are determined to pin Mao high in the pantheon of 20th-century villainy.

This massive biography traces Mao's life from his rural origins in Hunan province, detailing his hatred of his father and arguing that from the start he was a "lukewarm believer" in communism. The book contends that Mao grew into a young man without a moral code and obsessed with "upheaval and destruction."